The present invention relates to a method for the preparation of a bis(hydroxyphenyl)sulfides as an intermediates for the preparation of 4-mercaptophenol which in turn is a starting material for the synthetic preparation of, for example, 1,7-di(4-hydroxyphenylthio)-3,5-dioxaheptane as a color developing agent in heat-sensitive recording paper. More particularly, the invention relates to an efficient method for the preparation of a bis(hydroxyphenyl)sulfides capable of being reductively decomposed into a mixture of mercaptophenols containing 4-mercaptophenol in a greatly increased proportion relative to less useful 2-mercaptophenol by the reaction of phenol and sulfur chloride (S.sub.2 Cl.sub.2).
As is mentioned above, 4-mercaptophenol is a useful compound as a starting material of various sulfur-containing organic compounds. For example, 1,7-di(4-hydroxyphenylthio)-3,5-dioxaheptane as a color developing agent in heat-sensitive recording paper is synthesized by reacting 4-mercaptophenol with bis(2-chloroethoxy)methane while 4-mercaptophenol is prepared by reductively decomposing a bis(hydroxyphenyl)sulfides conventionally prepared by the method disclosed in U.S. Pat. No. 3,647,885 according to which phenol and elementary sulfur in admixture without any solvent are reacted at an elevated temperature in the presence of an alkali metal hydroxide as a catalyst. The product of this reaction is, of course, a mixture of various kinds of different bis(hydroxyphenyl)sulfides, from which the reductive decomposition gives a mixture of mercaptophenols containing the undesired 2-mercaptophenol in a large or major proportion so that this process is industrially not advantageous in respect of the productivity of 4-mercaptophenol.
An alternative method is disclosed in Japanese Patent Kokai No. 50-24233 for the preparation of bis(hydroxyphenyl)sulfides, according to which phenol and sulfur chloride are reacted in an organic solvent such as methyl alcohol, acetonitrile, 1,2-dimethoxy ethane, acetic acid and ethyl acetate. This method is, however, disadvantageous in respect of formation of chlorophenols as a by-product and low selectivity for the formation of the desired bis(hydroxyphenyl)sulfide. In addition, the reaction product obtained by this method contains a relatively large amount of monosulfides insusceptible to the reductive decomposition and, moreover, the reductive decomposition of the reaction product gives the desired 4-mercaptophenol in a relatively low yield with a large amount of undesired 2-mercaptophenol as a by-product.